A convolution model to convert some
absorption model into a partial covering absorption. If the absorption model is
M(E) then this is converted to (1-CvrFract) + Cvrfact * M(E). Note that when
specifying the model it is important to put parentheses in the right place. Let
this model be P(E) which we want to apply to an absorption model M(E) then use
the result to multiply an additive model A(E). The combined model should be
specified as (P*M)*A, not P*M*A or P*(M*A).